Key performance variables (KPVs) are quantitative measurements of an athlete's performance. By accessing and reviewing an athlete's KPVs, the athlete can significantly improve overall performance: KPVs allow the athlete to share performance information with coaches, record and track performance over time, and provide real-time feedback to the athlete.
In sports such as skiing and snowboarding, KPVs can describe the performance of jumps. The KPV of a jump may comprise: 1) the air-time (duration of the jump from take-off to landing), 2) the distance (horizontal displacement of the athlete from take-off to landing), 3) the height (maximum vertical height the athlete achieves), and 4) the degree of rotation (the rotation athletes achieved during jumps). These KPVs can be used to provide feedback and motivation to the athlete.
Currently available video-based or camera-based motion capture (MOCAP) approaches provide few quantitative variables. Additionally, these MOCAP techniques are restricted to indoor use or confined areas that are not suitable for motion capture of outdoor sports such as skiing, snowboarding, rollerblading and biking since these activities take place over large distances.
Inertial navigation systems (INS), on the other hand, are self-contained and thus can provide unconstrained accessibility to advanced motion and location information. INS is widely used in various MOCAP applications such as ship and aircraft navigation, fastening tool and pen tracking, and sports analysis to provide information such as attitude, velocity, and position.
Recently, miniature micro-electromechanical systems (MEMS) inertial devices have become more common, and the small size of MEMS inertial devices has resulted in the emergence of INS for human body motion tracking using wearable MOCAP technology. Typically, wearable MOCAP devices make use of a MEMS inertial measurement unit (MEMS-IMU) and/or an absolute position sensor to capture motion for indoor/outdoor localization. For example, a MOCAP camera system can act as an absolute position sensor and be added to inertial devices for more accurate human body localization in an indoor environment. For outdoor environments, a global positioning system (GPS) is an exemplary absolute position sensor that can be used to augment an inertial measurement unit for more accurate human body localization.
A disadvantage in the above mentioned GPS/MEMS-IMU integration approaches is that the consumer grade GPS-derived vertical positional (or altitude) information is typically much less accurate than the horizontal positional information. GPS altitude measurement accuracies can vary up to 40 m (with 10 m to 20 m being common); the most important reasons for this inaccuracy are satellite visibility (i.e. few visible satellites over the horizon) as well as multipath signal effects. Real-time GPS technologies such as real-time kinematic GPS and differential GPS provide higher positional accuracies but their prohibitive cost is a limiting factor for the sport consumer electronics market.